The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to an improved attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. In the most common configuration, the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically, resting on the lower support plate. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles in thereby forming an integral fuel assembly. The respective top and bottom nozzles extend slightly above and below the ends of the fuel rods, capturing the rods therebetween.
During operation of such assembly in a nuclear reactor, the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of an integral assembly of guide tubes welded to the top and bottom nozzles, its difficult to detect and remove the failed rods. To gain access to these rods, it is necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the control rod guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in a reactor because of the damage done to both the guide thimbles and the nozzles which prohibits rewelding.
In view of the high costs associated with replacing fuel assemblies, both domestic and foreign utilities indicated an interest in reconstitutable fuel assemblies in order to minimize their operating and maintenance expenses. The nuclear reactor industry responded by providing a reconstitutable fuel assembly with a removable top nozzle. There are different types of these reconstitutable fuel assemblies, but the majority employ a threaded arrangement for attaching the top nozzle to the control rod guide thimbles whereby the top nozzle can be removed to gain access to the fuel rods contained in the skeleton assembly.
One type of such reconstitutable fuel assembly can be seen in the Klumb et al patents (U.S. Pat. No. 3,770,583 or U.S. Pat. No. 3,814,667). Shown is a top nozzle of the type having a hold-down device incorporated therewithin and essentially includes coil springs coaxially disposed about upwardly extending alignment posts that are threaded through an end plate with fastener nuts located on the underside of the plate. An upper hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the end plate. A radially enlarged shoulder on the upper end of the alignment posts retain the hold-down plate on the posts. In an attempt to improve upon the Klumb et al device, Anthony et al set forth another threaded joint arrangement as seen in U.S. Pat. No. 3,992,259. Yet another type of threaded arrangement used for removably attaching the top nozzle on the control rod guide thimbles can be seen in U.S. Pat. No. 3,828,868.
These prior art reconstitutable fuel assemblies employing such threaded arrangements not only suffer from high manufacturing costs, complicated designs, difficlty in removing and reattaching the top nozzle, but, in addition thereto, it has been found that after irradiation, the threaded connection becomes corroded and inoperable, thus requiring some sort of destructive action to the nozzle and/or the guide thimbles resulting in costly damage to these components, yet alone rendering the fuel assembly unfit for further use.